Conventionally, the distributor-counterpart is a non-rotary part through which the fluid passes, in particular for feeding and discharging, before reaching the distributor, which distributes said fluid among the various distribution ducts that are put into communication one after the other with the cylinders of the cylinder block of the hydraulic machine. By way of example, for a stationary casing and rotary cylinder block machine, the distributor-counterpart is a casing portion; by way of example, for a rotary casing and stationary cylinder block machine, the distributor-counterpart is a stationary core.
Document EP 0 284 460 describes a pressurized fluid mechanism (motor or pump) having three operating cylinder capacities and having a distributor, which is an internal distributor, that presents a radial distribution face. That mechanism comprises two cylinder-capacity selectors, each arranged in the casing portion of the distribution device and each suitable for being controlled separately. That mechanism gives satisfaction, but it presents two drawbacks. Firstly, its radial size is considerable since it needs to be able to house two cylinder-capacity selection slides in two different bores that are provided in the casing portion of the distribution device. Secondly, it requires a distributor and a distributor-counterpart that are very specific, which must be specially provided for that mechanism, without either of them being suitable for use in a mechanism having only one cylinder-capacity or only two cylinder-capacities. Thus, for that mechanism it is not possible to make use of a simpler distributor or distributor-counterpart, as used for other mechanisms. However, although it is conventional for each mechanism of the type described in EP 0 284 460 to provide a specific internal distributor adapted to the looked-for after functions, it is desirable for reasons of economy of production to be able to maximize use of the same distributor-counterpart for other mechanisms (which distributor-counterpart is specifically a casing portion surrounding the internal distributor), regardless of their numbers of cylinder capacities.
Document FR 2 127 268 also discloses a hydraulic mechanism having three operating cylinder capacities. In that mechanism, a cylinder-capacity selection device is arranged in the distributor, which is an internal distributor. That mechanism comprises a slide that is capable of adopting three positions: a first position obtained by feeding a first control chamber with fluid; a second position obtained by feeding another control chamber opposite the first; and a third position obtained without feeding the control chambers, but under the effect of the return spring. That mechanism makes it possible to overcome the above-mentioned problem in part since the cylinder-capacity selection device is arranged in the internal distributor. However, the shape of the cylinder-capacity selection slide is complex. In addition, the distribution ducts provided in the internal distributor are complex in shape, so that the ducts can open out in the proximity of one another in the various regions of the bore in which the single cylinder-capacity selection slide moves. Thus, it is a mechanism in which the distribution face is an outer axially-extending face of the internal distributor, said internal distributor being inserted like a core inside the cylinder block. Because of the complexity of the shape of the distribution ducts, that mechanism does not seem compatible with a distributor having a radial distribution face. Consequently, the mechanism also needs to be of considerable size radially and, de facto, it is usable only with axial distribution face technology, which presents certain drawbacks, in particular in terms of sealing between the axial distribution face and the axial communication face of the cylinder block into which the cylinder ducts open out in order to be put into communication with the distribution ducts.